Metal die casting processes, such as the squeeze casting process described in U.S. Pat. No. 5,355,933, are well known in the art. Molten casting metal, usually aluminum or aluminum alloys, is introduced into a die and a great amount of pressure is exerted on the metal as it solidifies to essentially squeeze-form the article in the die producing a metal article having a lower porosity and improved mechanical properties as compared to the same article produced by more conventional lower pressure casting methods. In order to provide a hollow or cavity in the pressure cast article, a melt-out core is inserted into a die cavity prior to casting the molten metal in the die. The melt-out core, preferably coated with insulating mica material and a graphite release agent, is supported in the die cavity of a high-pressure squeeze casting apparatus. Molten casting metal having a melting temperature above that of the core is injected into the die and pressurized to about 15,000 psi to squeeze-form the article in the die and around the core. The molten casting metal is allowed to solidify in the die cavity and about the insulated core. The core preserves a space defining the hollow or core in the cast article. The resultant aluminum cast article and core are then heated to above the melting temperature of the core. The core material is then melted or liquified and extracted from the article leaving the aluminum article having a hollow.
In high-pressure aluminum casting processes, zinc melt-out cores have been described for use in forming an aluminum cast article having a cavity. In these processes, the zinc melt-out core is used as described above to preserve a space in the casting process corresponding to the cavity in the aluminum cast article. One significant problem with these high-pressure aluminum casting processes is the inability of the zinc core to maintain its integrity throughout the casting process. That is, during the casting process, the zinc core material seeps into the aluminum material forming the pressure cast article. This is believed to be caused by either capillary diffusion of the zinc when the aluminum solidifies, or by the zinc expanding when it melts when molten aluminum is cast about the zinc core. Thus, zinc melt-out cores do not provide sufficient "hot strength" to maintain its integrity during an aluminum pressure casting process. As such, the zinc seeps into the aluminum article during the casting process. This results in potential points of failure in the aluminum cast article which are unacceptable.
To address this problem, the prior art has applied various outer coatings to the zinc melt-out core in an attempt to maintain the core's integrity during the casting process. Such core materials include refractory materials including aluminum oxide, water glass, mica and graphite. These materials have not been completely successful and add additional cost to the casting process. Thus, there has been a need for an improved pressure casting process which results in an aluminum cast article having a hollow.